Major methods widely used to produce a plastic film (hereinafter referred to simply as a film) from the melt include a step for discharging a molten resin continuously and bringing it into contact with a cooling roller to achieve its cooling and solidification (hereinafter referred to as casting step) to provide a web-like plastic film. In some of them, the casting step, for example, is not followed by a step for stretching the film in the flow direction (hereinafter referred to as stretching step) in order to produce film that is not stretched (hereinafter referred to as unstretched film). In such cases, the product manufacturing speed is equal to the speed of the cooling roller and accordingly, there are particularly strong demands for high-speed rollers that can ensure improved productivity.
In the casting step, on the other hand, components, such as low molecular weight polymers and monomers including oligomers and additives, are volatilized from the molten resin and then cooled and solidified on the cooling roller, leading to gradual contamination of the surface of the cooling roller. As the surface of the cooling roller is contaminated, the molten resin may fail to be cooled sufficiently, causing troubles such as deterioration in appearance quality and physical properties, or accumulated contaminants may come off and adhere to the film. If the casting step is accelerated, it is naturally expected that the discharge rate of the molten resin will increase and accordingly larger amounts of the volatile components will be volatilized, making the above troubles more serious. Consequently, frequent cleaning of the cooling roller will be required and it will become difficult to continue the film production process for a long period of time.
To solve this problem, it has been proposed that the molten resin in contact with the cooling roller or the film cooled and solidified be pressed between the cooling roller and a press roller having a grooved surface so that contaminants on the cooling roller are removed continuously by the film as they are generated, thus achieving continuous cleaning of the cooling roller. (Patent document 1 for example) Such a press roller is generally referred to as a sweeper roller and accordingly also called so hereinafter.
When the molten resin and the cooling roller come into contact with each other, the move of the air caught between the molten resin and the cooling roller is arrested at the contact point between them to form air pockets (hereinafter referred to as air banks) that can cause creasing and breakage of the film. To prevent this from occurring, the grooves in the surface of the sweeper roller are designed to facilitate the leak of air.
However, when a conventional sweeper roller as proposed in Patent document 1 is used, air banks may be formed if the casting rate in the casting step is too high or if the layer of the molten resin is too thick. If the proportion of the grooves to the surface area of the sweeper roller is increased in an attempt to prevent the formation of air banks, furthermore, the shapes of the grooves in the sweeper roller are transferred strongly to the film, leading to defects called sweeper marks.